The present invention relates to improved processes for producing acicular magnetic metallic particle powder, more particularly to novel processes for producing the same most suitable for a magnetic material used for magnetic recording, which has a large saturated magnetic flux density .delta.s (e.g. 90-200 emu/g) and a high coercive force Hc (e.g. 500-2000 Oe) and is in such a condition that no cross linking between the particles exists and so the particles are substantially independent from each other.
In recent years, a demand for a high efficiency of magnetic recording media has more and more increased with the progress in miniaturizing and lightening a reproducing apparatus for magnetic recording. Namely, it has been demanded to elevate a bit density, an output characteristic and especially a frequency characteristic of magnetic recording media. Therefore, a magnetic recording material must have a large saturated magnetic flux density and a high coercive force to satisfy the said demand.
By the way, the magnetic materials conventionally employed in magnetic recording media are magnetic metal oxide powder such as magnetite, maghemite and chromium dioxide, and each of these magnetic oxide type powder has 70-85 emu/g of saturated magnetic flux density .delta.s and 250-500 Oe of coercive force Hc. And it is a main factor in limiting the level of reproducing output and bit density that .delta.s of above magnetic oxide type particle powder is at most about 85 emu/g and generally 70-80 emu/g. Further, Co-magnetite or Co-maghemite magnetic powder, having been also used as a magnetic recording material, is characterized by a coercive force Hc as high as 400-800 Oe and on the contrary a saturated magnetic flux density .delta.s as low as 60-70 emu/g.
On the other hand, instead of these magnetic oxide type powder, a development of magnetic non-oxide type particle powder having such properties as a larger saturated magnetic flux density and a still higher coercive force suitable for recording with both of high reproducing output and bit density, has been recently promoted. An acicular magnetic iron particle powder is one of examples having such properties as abovementioned.
A process generally known in the prior art to produce the acicular magnetic iron particle powder comprises reducing acicular iron (III) oxide hydroxide particles of acicular ferric oxide particles at a temperature as lower as possible than 350.degree. C. in a stream of reducing gas. In the above-mentioned process the higher the heating temperature at the reduction in reducing gas, the larger the saturated magnetic flux density .delta.s of acicular magnetic iron particle powder becomes. It is, however, ascertained that a deformation of the acicularity of this resultant magnetic iron particle powder and a sintering between said particle powder remarkably proceed in that degree. And, therefore, the coercive force Hc of the obtained acicular magnetic iron particle powder becomes extremely low, since the coercive force of acicular magnetic iron particle powder used as a magnetic recording material largely depends on a shape-anisotropy thereof. An acicularity of magnetic iron particle powder is one of the most important properties. Accordingly, in a process for producing acicular magnetic iron particle powder, it is important to form first of all as a starting material acicular, iron (III) oxide hydroxide particles or acicular ferric oxide particles having superior acicularity. After the formation of said particles, there arises a problem how to keep this acicularity in reducing the same while heating to produce acicular magnetic iron particle powder.
The shape of the particle is especially sensitive to the heating temperature, and the particle growth is so remarkable, particularly in reducing atmosphere, that the unit particle grows over the original size of the particle itself and the external shape of the particle itself is gradually deformed to cause a modification of the shape and a sintering between the particles. As the result, the coercive force is lowered. Thus, there has been arisen a serious difficulty that the condition for obtaining acicular magnetic iron particle powder having a high coercive force Hc, namely maintaining the low heating temperature when reducing, conflicts with the condition for obtaining acicular magnetic iron particle powder having a large saturated magnetic flux density .delta.s, namely maintaining the high heating temperature when reducing.
On the other hand, the following processes are provided to prevent the sintering of magnetic particles in a process for producing magnetic oxide type particles such as acicular magnetic ferric oxide by means of laying SiO.sub.2 on the particles (Japanese patent application laying open Nos. 83100/73 and 41299/74).
(A) A process comprising adjusting a suspension containing acicular iron (III) oxide hydroxide particles or acicular ferric oxide particles to an acid region, pH 4.0-6.5, and adding water-soluble silicate thereinto;
(B) A process comprising adjusting a suspension containing acicular iron (III) oxide hydroxide particles or acicular ferric oxide particles to over about pH 12 and adding water-soluble silicate thereinto while treating said particles through a hydrolysis of water-soluble silicate under oxidizing atmosphere.
However, the inventors of the present invention found that the above-mentioned processes (A) and (B) did not remove the aforementioned difficulty in producing acicular magnetic iron particle powder of non-oxide type. That is, as the result of thorough research, it is found that these known processes for laying SiO.sub.2 on magnetic particles have following defects.
In the process (A), as soon as water-soluble silicate is added into the suspension, an immediate precipitation in the form of SiO.sub.2 arises and it is easy to form a mixture of ferric oxide particles and SiO.sub.2 particles. Accordingly, SiO.sub.2 particles become to lie on the surface of ferric oxide particles unevenly, and the absorbability between SiO.sub.2 particles and ferric oxide particles is weak. And, in the process (B), in the case that sodium silicate is used as water-soluble silicate, water-soluble sodium silicate is hydrolyzed to produce Na.sub.2 Si.sub.2 O.sub.5 as shown in formula (1) below, and then Na.sub.2 Si.sub.2 O.sub.5 is decomposed by the dissolved oxygen or oxygen gas to form a precipitation of SiO.sub.2 particles as shown in formula (2) below. The resultant precipitated SiO.sub.2 particles lie on the magnetic particles. EQU 2Na.sub.2 SiO.sub.3 + H.sub.2 O .revreaction. Na.sub.2 Si.sub.2 O.sub.5 + 2NaOH (1) EQU na.sub.2 Si.sub.2 O.sub.5 + 1/2O.sub.2 .fwdarw. 2SiO.sub.2 + 2NaO (2)
However, this process for laying prepared SiO.sub.2 particles on iron (III) oxide hydroxide particles through hydrolyzing water-soluble sodium silicate results in that the obtained SiO.sub.2 particles laid on the magnetic particles are uneven and coerse similarly in the process (A) because of taking long time for the hydrolysis.
Further, in the both of said processes (A) and (B), the amount of SiO.sub.2 actually laid on acicular iron (III) oxide hydroxide particles is just a little comparing with that of added water-soluble silicate calculated as the amount of SiO.sub.2.
In fact, as shown in each example in the Japanese patent application laying open Nos. 83100/73 and 41299/74 the amount of SiO.sub.2 laid on acicular iron (III) oxide particles equals to 2-3 wt.% of added water-soluble silicate calculated as the amount of SiO.sub.2. This is because that the resultant SiO.sub.2 precipitate is in the suspended state in the solution without being laid on acicular iron (III) oxide hydroxide therein to form a mixture of acicular iron (III) oxide hydroxide particles and SiO.sub.2 particles, which are undesirable from the economical point of view, since most of the resultant SiO.sub.2 particles do not lie on the surface of iron (III) oxide hydroxide particles and are in the state of the uneffective suspension.